In the field of medicine, an implant is deemed to have failed if it fails to meet the claims of its manufacturer or the health care provider involved in its installation. Basically, implant failure can have any number of causes that may be mechanical, physical, chemical or biological. At present, mechanically assisted crevice corrosion (MACC) has been associated with implant failure in vivo and is a serious concern in numerous metallic implant systems. For instance, stainless steel medical devices may be subjected to fretting and crevice corrosion in the human body as are titanium and CoCrMo alloys due to the presence of a passive oxide film on their surface. At present, AISI 316L stainless steel has been widely used as a metallic biomaterial for orthopaedic, spinal, dental and cardiovascular implants. Consequently, crevice corrosion has been a serious concern for stainless steel implants.
One mechanism of MACC that has not been clearly identified and studied is fretting-initiated crevice corrosion (FICC) of stainless steel where an initial fretting event can initiate a rapid propagating crevice corrosion process even when fretting has ceased. In fact, a review of published work reveals that no systematic study has been conducted to investigate the mechanism of fretting initiated-crevice corrosion. On this account, researchers from the Clemson – MUSC Bioengineering Program, Department of Bioengineering, Clemson University and the Medical University of South Carolina: Dr. Yangping Liu (postdoctoral fellow), Dongkai Zhu, David Pierre and led by Professor Jeremy Gilbert proposed to investigate FICC behavior of stainless steel in physiologically representative saline solution in an in vitro pin-on-disk fretting corrosion model. Their work is currently published in the research journal, Acta Biomaterialia.
To begin with, the voltage-dependent fretting corrosion behavior of stainless steel pins in contact with stainless steel disks immersed in a physiologically representative phosphate buffered saline was studied. Fretting corrosion experiments were conducted with the goal being to understand both the mechanical and electrochemical conditions needed for FICC using a customized pin-on-disk fretting corrosion system. The team used a customized 2-D pin-on-disk fretting corrosion system, interfaced with a digital optical microscope to directly observe, in real time, the surface contact geometry and corrosion product generation during fretting and crevice corrosion.
The authors observed that crevice corrosion could be induced by fretting at potentials as low as −100 mV. To be specific, the researchers established that below −100 mV, there was no FICC, but rather fretting corrosion stopped immediately after fretting ceased and returned to a stable baseline current. On the contrary, metastable FICC was shown at potentials between −100 mV and 0 mV, when the crevice corrosion current gradually decreased over several seconds or longer after fretting ceased. Self-sustained, unstable crevice corrosion started at 50 mV, where prior to fretting the currents were low, and after just a few cycles of fretting the crevice current rose rapidly and continued to increase after fretting stopped.
In summary, the study demonstrated the process of fretting-initiated crevice corrosion in 316L stainless steel in simulated physiological solution of phosphate buffered saline. Overall, the results demonstrated that fretting initiated crevice corrosion may affect the performance of stainless steel in vivo. In a statement to Advances in Engineering, Professor Jeremy Gilbert explained that their findings clearly established the fundamental differences between the FICC mechanism and conventional crevice corrosion theory, showing that fretting can play a significant role in the initiation of crevice corrosion of stainless steel.
Yangping Liu, Dongkai Zhu, David Pierre, Jeremy L. Gilbert. Fretting initiated crevice corrosion of 316LVM stainless steel in physiological phosphate buffered saline: Potential and cycles to initiation. Acta Biomaterialia, volume 97 (2019) page 565–577.